The inverter in a solar photovoltaic power generation system recently trends to employ distributed micro inverters (micro-inverters). The micro-inverter may provide maximum power point control for each direct current (DC) photovoltaic assembly, such that each DC photovoltaic assembly can produce a maximum energy, thereby improving the performance of the whole solar photovoltaic power generation system. Furthermore, the micro-inverter may also produce an alternating current (AC) low voltage output, rather than the high DC voltage output as produced by a centralized inverter system, so that the security and efficiency of the system can be improved.
In a photovoltaic power generation system that uses micro-inverters, each of a plurality of branches is coupled with a string of micro-inverters and other related components via a distribution box. A large photovoltaic power generation system is typically coupled using a three-phase AC power grid. However, a real three-phase micro-inverter will have much more electronic elements than a single-phase micro-inverter, and thus is rather complicated to implement. An existing alternative method is to connect a plurality of single-phase micro-inverters in series as three groups, with each group of single-phase micro-inverters being further coupled to one phase of a commercial three-phase AC power grid. In order to ensure the balance of the current amplitudes between the three phases of the AC power grid, the three groups of single-phase micro-inverters need to have the same micro-inverters and DC photovoltaic assemblies. This requirement causes difficulty in the design and installation as well as high cost of the solar photovoltaic power generation system.
FIG. 1 is a schematic block diagram of a solar photovoltaic power generation system in the prior art. The solar photovoltaic power generation system 100 comprises a plurality of single-phase micro-inverters 101 connected in series as several groups (three groups are illustrated), which are coupled with a three-phase AC power grid, respectively. Each single-phase micro-inverter 101 generates a current having the same phase as the corresponding voltage of the three-phase AC power grid 106, and thus a grid-connected output can be obtained. Each single-phase micro-inverter 101 may be coupled with a DC photovoltaic assembly 102 via a DC terminal 103, to convert the DC current generated by the DC photovoltaic assembly 102 to an AC current. Each single-phase micro-inverter 101 may be coupled with a previous/next single-phase micro-inverter 101 via AC terminals 104, so that the plurality of single-phase micro-inverters 101 in each row may be coupled in series as a branch. The AC terminals 104 of the last single-phase micro-inverter 101 of each branch may be used as AC output terminals of the entire branch, and may be coupled to the neutral wire N and one of the live wires L1, L2, or L3 of the three-phase AC power grid 106. Accordingly, the three branches as shown may be coupled to L1/N, L2/N, and L3/N, respectively.
In the operation of photovoltaic power generation, the system 100 may couple the generated AC currents via a distribution box to the three-phase AC power grid 106, such as a commercial 220V/380V power grid. Consequently, the first phase current of the AC currents may be carried by the live wire L1 of the three-phase AC power grid 106, the second phase current of the AC currents may be carried by the live wire L2, and the third phase current of the AC currents may be carried by the live wire L3. The current carried by the neutral wire N is the sum of the three phase currents. Ideally, the magnitudes of the currents on the live wires L1, L2, and L3 are balanced, with a phase difference of 120 degrees, so that the current on the neutral wire N shall be zero. However, in order to achieve the balance between the three phases of the three-phase AC power grid 106, balanced currents should be generated in the three branches of the photovoltaic power generation system 100 as shown. That is, the three groups of DC photovoltaic assemblies 102 and the three groups of single-phase micro-inverters 101 should generate currents having the same magnitude. If the system 100 is rather large in scale, it will be quite difficult to ensure that the environment conditions where the various DC photovoltaic assemblies 102 are located, such as illumination, temperature, etc., are the same, thus it will be very difficult to maintain the balance of the AC currents between the three phases of the input power grid 106, which causes difficulty in the design and installation as well as high cost of the photovoltaic power generation system 100.